1. Field of the Invention
The present invention relates in general to injection molding systems. More particularly, the invention relates to a hot runner nozzle having a heated tip.
2. Discussion of the Related Art
A primary concern in injection molding is the necessity to keep the material melt flowing at its optimum processing temperature until it enters the gate to the cavity of the cooled mold where the plastic product is formed. If the melt temperature is too high, the melt can burn or degrade resulting in a poor quality product. If the melt temperature is too low the melt flow can be retarded or even freeze up, and visible imperfections in the molded product may result.
As the melt travels down the length of the hot runner nozzle, it nears the gate where the melt exits the heated nozzle and enters the cooled mold. Heat loss from the nozzle to the mold is greatest in the gate area. Further, the end of the nozzle can contact the mold to form a seal about the gate area to prevent melt form leaking about the nozzle body. This contact area can cause further heat loss from the nozzle to the mold. The heat loss can result in a reduction in melt temperature near the gate area, and the consequent problems mentioned above can occur. Nozzle heaters such as coil heaters disposed about the nozzle are used to keep the nozzle, and melt traveling therethrough, at a uniform, constant temperature. However, these heaters do not typically extend past where a nozzle insert can be seated in the nozzle body. Thus, in the area where the melt is most susceptible to a reduction in temperature due to the proximity of the mold, there is no external heating present.
The problem of heat loss about the gate is magnified when dealing with a material with a small processing window, such as semi-crystalline materials, in which the difference between the processing temperature and the no-flow temperature of the plastic can be as small as 30.degree. F. If the end of the nozzle near the gate area runs too cold the melt can freeze in the insert during cycle interruptions. If this happens, it is necessary to apply external heat to the tip to unfreeze the melt, such as a blow torch. Further, material left in the insert between cycles can cool and during the next injection cycle, the cold material can contact a side wall of the mold cavity appearing as blush in the molded part. The blush will show as an unacceptable imperfection in the finished product.
Another problem which arises due to heat loss about the gate occurs in multiple nozzle, single cavity applications. In such applications, multiple nozzles work in conjunction to simultaneously inject melt into a single cavity. This arrangement is typically used for complex, thin walled products, such as automotive battery containers. In this application balanced flow of the nozzles into the cavity is critical. Between injection cycles the plastic will freeze off in the gate forming a thin layer of frozen plastic. Upon initiation of injection the pressure exerted on the gate will blow the frozen plastic into the cavity and remelt it. Cores of the mold cavity separating the walls of the product can shift if the plastic does not flow evenly and simultaneously about the six cores. If the temperatures at the gate and in the insert vary, the width of the layer of frozen plastic will vary from nozzle to nozzle. The thicker the layer, the more pressure it takes for the nozzle to begin extruding melt into the cavity. Thus, different starting times and consequent uneven melt flow results. This can result in unacceptable, and uneven cavity wall thickness due to core shifting. One method currently used to address this problem is to vary the bore diameters in each nozzle to compensate for uneven material flow. However, it is tedious and time consuming to change the bore diameter, and variations in the material or process can alter the flow balance.
Accordingly, it is one object of the present invention to maintain the hot runner nozzle at a uniform temperature down its length to keep the melt at a constant and uniform temperature.
It is another object of the present invention to provide additional heat at the insert where heat loss to the mold is greatest.
It is another object of the present invention to provide a multiple injection single cavity system that has a balanced flow.
It is another object of the present invention to provide a tip heated hot runner nozzle having a variety of different types of inserts including free flow, cone shaped, spiral, and valve pin.